1. Field of the Invention
The present invention relates to a light obstruction type smoke sensor (hereinafter, referred to merely as a "sensor") which detects attenuation of light arriving at a light receiving unit from a light generating unit through a space of a monitoring area, and which senses an existence of smoke caused by a fire or the like, and more particularly to a control of the quantity of light of the light generating unit.
2. Description of the Related Art
FIG. 2 is a diagram showing the configuration of a conventional sensor.
The sensor has two terminals 1a and 1b for connecting the sensor to a receiver or a repeater via sensor lines which are not shown. The sensor receives a power supply of DC 24 V from the receiver or the repeater via the sensor lines.
A warning circuit 2 and a constant voltage circuit 3 are connected to the terminals 1a and 1b. The warning circuit 2 short-circuits the terminals 1a and 1b based on an alarm signal ALM, and causes a current of a predetermined level or higher to flow through the sensor lines. The constant voltage circuit 3 generates a stabilized supply voltage VP of DC 10 V which is required for circuits in the sensor, from the power source of DC 24 V supplied via the sensor lines.
A light generating circuit 4, a light receiving circuit 5, and a smoke detection processing circuit 6 are connected to an output side of the constant voltage circuit 3. The light generating circuit 4 consists of a light-emitting diode (hereinafter, referred to as an "LED") and the like, and emits light of constant luminance. The light receiving circuit 5 consists of a phototransistor and the like. The light receiving circuit receives the light which arrives from the light generating circuit 4 through a space of a monitoring area, and outputs an analog voltage corresponding to the received light level.
The smoke detection processing circuit 6 consists of an analog/digital converter (hereinafter, referred to as an "A/D") 6a and a microprocessor (hereinafter, referred to as an "MPU") 6b. The analog voltage corresponding to the received light level is supplied from the light receiving circuit 5 to an input side of the A/D 6a. The A/D 6a converts the input analog voltage to a digital value, and outputs the digital value. An output side of the A/D is connected to the MPU 6b. The MPU 6b has a function of monitoring the received light level of the light receiving circuit 5, and also has a function of, when an abrupt change of the received light level is detected, outputting an alarm signal ALM. The alarm signal ALM output from the MPU 6b is supplied to the warning circuit 2.
In the sensor, the supply voltage VP of 10 V is generated by the constant voltage circuit 3 from the voltage of DC 24 V supplied via the sensor lines, and then supplied to the light generating circuit 4, the light receiving circuit 5, and the smoke detection processing circuit 6. Accordingly, light of a constant level is output from the light generating circuit 4, and the light enters the phototransistor of the light receiving circuit 5 through the space of the monitoring area. If any shielding material such as smoke does not exist in an optical path between the light generating circuit 4 and the light receiving circuit 5, an analog voltage of a predetermined level is obtained in the light receiving circuit 5. The analog voltage obtained in the light receiving circuit 5 is converted into a digital value in the A/D 6a and then supplied to the MPU 6b.
The MPU 6b periodically monitors data of received light levels supplied from the A/D 6a, and compares the data with a reference value which is obtained from an average value of several sets of preceding data. If the difference between the new data and the reference value is equal to or lower than a predetermined allowable variation width, it is not judged that a fire occurs, and the reference value is corrected by the new data.
If the new data is lowered so as to exceed the allowable variation width, it is judged that a fire occurs, and an alarm signal ALM is output from the MPU 6b to the warning circuit 2. When the alarm signal ALM is supplied to the warning circuit 2, the warning circuit 2 short-circuits the terminals 1a and 1b with a given impedance. Accordingly, a predetermined short-circuit current flows through the sensor lines, so that the alarm can be detected by the receiver or the repeater on the side of the power supply.
However, the above-described conventional sensor has the following problems.
The light generating circuit 4 is driven by the constant supply voltage VP of DC 10 V supplied from the constant voltage circuit 3, so that light of constant luminous intensity is output. On the other hand, a gentle variation of the received light level of the light receiving circuit 5 is corrected in the smoke detection processing circuit 6, and a fire is judged by using the corrected average received light level as a reference value.
Such a correction is performed only in the light receiving side. Accordingly, if the received light level is gradually lowered by a reason such as deterioration of the LED, deviation of the optical path between the light generation side and the light receiving side and caused by deformation of the building or the change of installing conditions, or a soiled reflecting mirror in the optical path, the judgment on a fire is performed by using the lowered received light level as the reference value. When the received light level which will be used as the reference value is gradually lowered, therefore, the signal-to-noise ratio is deteriorated and there occurs deviation in sensitivity, so that it may be impossible to accurately perform the judgment on a fire.
In the installation of a sensor, it is necessary to set initial conditions in view of the variation in the quantity of receiving light. This produces a problem in that the allowable range which is set as an initial value of the received light level is narrow and hence the adjustment requires a prolonged time period.